Alzheimer's disease (AD) represents one of the greatest health care burdens, with 35 million affected individuals worldwide, a population estimated to increase to 115 million by 2050. [Wimo, Alzheimer's Disease International World Report 2010. The Global Economic Impact of Dementia, Alzheimer's Disease International (2010).] AD is a devastating dementia that first presents as progressive memory loss and later can include neuropsychiatric symptoms such as depression, paranoia, agitation and even aggression. Currently, available AD treatment is limited to cognitive enhancers with limited and short-lived efficacy.
Previously, diagnosis of AD could only be confirmed at autopsy by the presence of amyloid deposits and neurofibrillary tangles (NFTs) containing the microtubule-associated protein tau. Current clinical diagnoses of AD satisfy the DSM-IV TR and the NINCDS-ADRDA Work Group criteria for probable AD in McKhann et al., Neurology 34(7):939-944 (1984). Initial diagnostic criteria based mostly on subjective assessments set out in McKhann et al., above, require that the presence of cognitive impairment and a suspected dementia syndrome be confirmed by neuropsychological testing for a clinical diagnosis of possible or probable AD; although they need histopathologic confirmation (microscopic examination of brain tissue) for the definitive diagnosis.
The criteria specify as well eight cognitive domains that may be impaired in AD. Those cognitive domains that may be impaired in AD are: memory, language, perceptual skills, attention, constructive abilities, orientation, problem solving and functional abilities. These criteria have shown good reliability and validity, and are those used herein as the basis for assertion of clinical diagnosis of AD.
Clinical criteria for the diagnosis of Alzheimer's disease include insidious onset and progressive impairment of memory and other cognitive functions. There are no motor, sensory, or coordination deficits early in the disease. The diagnosis could not heretofore be determined by laboratory assays. Such assays are important primarily in identifying other possible causes of dementia that must be excluded before the diagnosis of Alzheimer's disease can be made with confidence. Neuropsychological tests provide confirmatory evidence of the diagnosis of dementia and help to assess the course and response to therapy. The criteria proposed by McKhann et al. are intended to serve as a guide for the diagnosis of probable, possible, and definite Alzheimer's disease; these criteria will likely be revised as more definitive information become available.
Diagnostic criteria have more recently been refined to include the prodromal phase (early symptoms that occur before the full-blown symptoms of the disease hit) termed “Mild Cognitive Impairment (MCI) due to AD.” This new diagnosis reflects a desire to treat the disease earlier because the neuropathology is estimated to start 10 years prior to appearance of symptoms. [Trojanowski et al., Alzheimers Dement 6, 230-238 (2010)] Clinical trials of potential disease-modifying treatments have been hugely disappointing, possibly in part because even an “early-stage” patient already has a massive amyloid-beta (Aβ) burden and substantial pathologies with significant synaptic defects and inflammation.
According to Petersen et al., Arch Neurol 56(3):303-308 (1999), the primary distinction between control subjects and subjects with MCI is in the area of memory, whereas other cognitive functions are comparable. However, when the subjects with MCI were compared with the patients with very mild AD, memory performance was similar, but patients with AD were more impaired in other cognitive domains as well. Longitudinal performance demonstrated that the subjects with MCI declined at a rate greater than that of the controls but less rapidly than the patients with mild AD.
Patients who meet the criteria for MCI can be differentiated from healthy control subjects and those with very mild AD. They appear to constitute a clinical entity that can be characterized for treatment interventions.
Amyloid-beta (Aβ), a peptide of 39-42 amino acids that is generated in vivo by specific, proteolytic cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. Aβ42 comprises residues 677-713 of the APP protein, which is itself a 770-residue transmembrane protein having the designation P05067 in the UniProtKB/Swiss-Prot system. Aβ, and in particular the Aβ42, is commonly believed to be the principal causative agent in AD, although its mechanism underlying AD neuropathologies is debated.
Cognitive impairment and the magnitude of synaptic deficit in AD brain is more highly correlated with soluble Aβ than with the abundance of amyloid plaques, suggesting it is the soluble Aβ that causes active impairment. [Naslund et al., JAMA 283, 1571-1577 (2000)] Robust evidence demonstrates that soluble Aβ can elicit a toxic signaling cascade via the α7nAChR leading to impaired synaptic activities and subsequent Aβ42 intraneuronal aggregates and cognitive deficits. [Wang et al., J Neurosci 35, 10961-10973 (2009); Liu et al., PNAS 98, 4734-4739 (2001); Pettit et al., J Neurosci 21, RC120-RC125 (2001); Chen et al., Neuropharmacology 50, 254-268 (2006); and Dziewczapolski et al., J Neurosci 29, 8805-8815 (2009)] Aβ42 binds the α7nAChR with extraordinarily high (high femtomolar) affinity [Wang et al., J Biol Chem 275, 5626-5632 (2000); and Wang, et al., J Neurochem 75, 1155-1161 (2000)] and this interaction activates the kinases ERK2 and JNK1, which phosphorylate tau protein, leading to the formation of neurofibrillary tangles (NFTs). [Wang et al., J Biol Chem 278, 31547-31553 (2003)]
Pharmacotherapies have attempted to disrupt Aβ42-elicited toxic signaling by preventing Aβ42 from binding to α7nAChRs, a difficult task for two reasons: first, surpassing the sub-picomolar affinity of the interaction requires an even higher affinity interaction, and second, directly targeting the receptor with chronic receptor agonists or antagonists can alter its sensitivity or cell surface expression level. In fact, the desensitization of acetylcholine receptors following enhanced and prolonged stimulation due to administration of acetylcholinesterase inhibitors, the only approved therapy for AD, is thought to be the primary reason that this class of drugs provides only short-lived cognitive enhancement.
The present invention utilizes that sub-picomolar interaction (protein-protein complex formation) between Aβ42 and α7nAChR, and similar newly found protein associations (complexes), between α7nAChR and FLNA, and also TLR4 and FLNA as bases for a contemplated objective assay that can diagnose the presence of AD in a living person using the relatively non-invasive technology of body sampling. In another aspect, those same Aβ42-α7nAChR, α7nAChR-FLNA, and TLR4-FLNA interactions are utilized as bases for a contemplated objective prognostic and biomarker assay that can indicate the prognosis of treatment as well as track disease progression and treatment efficacy in a living, presumed AD patient using the relatively non-invasive technology of body sampling and a compound that exhibits particular binding activity similar to those of a compound used for treatment.